The objectives of this research project are to assess component skills in reading and spelling, related language skills, and perceptual skills in twins selected for deficits or normal function in reading and/or mathematics. Behavior-genetic analyses will be used to assess the genetic and environmental etiology for deficits in different component reading, language, and perceptual skills. Confirmatory factor analyses of twin data will be used to assess the genetic and environmental covariance among the component skills. A general goal is to separately identify deficits in component skills that are predominantly due to genetic factors and deficits that are predominantly due to environmental factors. Behavior-genetic analyses will also be used to validate subtypes and/or dimensions of individual differences in reading disabled children by assessing their differential genetic and environmental etiology. Children in identified subtypes will be studied for subtype-by-treatment interactions in Research Project V. Longitudinal data will be collected to assess subtype stability and differential prognosis in the development of reading skills.